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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

High-frequency RCS of open cavities with rectangular and circular cross sections

The radar cross-section (RCS) analysis of open-ended cavities with rectangular and circular cross sections is carried out using the waveguide modal approach and the shooting-and-bouncing ray (SBR) approach. For a cavity opening on the order of ten wavelengths or larger, the comparison between the two approaches is excellent. It is also observed that at lower frequencies the SBR results deviate from the more accurate modal results. On the other hand, the SBR approach allows for greater flexibility in geometrical modeling, and can be applied to problems where waveguide modes cannot be easily found. SBR results for an offset rectangular cavity and a circular cavity with rounded endplate are presented.

Ling, Hao↗
NASA NTRS1990

Computing Scattering Matrices For Circular Waveguides

Scattering Matrix Program for Circular Waveguide Junctions computes scattering matrix for series of circular waveguide sections. Sections must have same axis, but radius and length of each section completely arbitrary. Devices analyzed include simple waveguide step discontinuity like that used in a dual-mode horn, stepped matching section, or corrugated waveguide section with constant or varying slot depth. Certain types of corrugated horns also analyzed with program. Mathematical model used in program accurately predicts reflection and transmission characteristics of such devices, taking into account excitation of modes of higher order as well as multiple reflections and energy stored at each discontinuity. Written in FORTRAN 77.

Hoppe, Daniel J.↗
NASA NTRS1990

Theory of circular dielectric waveguide with anisotropic sheet cover

A circular dielectric waveguide consisting of an isotropic core covered with a thin anisotropic sheet is considered. The sheet is represented as a jump immittance and Maxwell's equations are applied. Solution of the boundary value problem yields the characteristic equation, or dispersion relation, which is then solved numerically. The results are verified for the step-index fiber and circular, metallic waveguides. Finally, examples are included to investigate the effects of the anisotropic sheet.

Kish, Jerome Thomas↗
NASA NTRS1989

A circular combustor configuration with multiple injection ports for mixing enhancement

A circular combustor design by Ghorashi (1988) which resembles a continuously-stirred tank reactor with multiple injection ports is presented with a view to the enhanced control of mixing, NO(x) reduction, and combustion efficiency maximization. Attention is given to the prototype apparatus for this type of circular combustor, which takes the form of a transparent cold-flow reactor for flow visualization studies under 'chemically frozen' conditions.

Ghorashi, B.↗
NASA NTRS1990

Spectra of circularly polarized radiation from astrophysical OH masers

A striking feature of astrophysical masers is the tendency for either one or the other of the circular polarizations to dominate in the radiation from the strong, widely observed masing transitions of OH at 18 cm. Spectral line profiles are calculated for polarized maser radiation due to the combined effects of a velocity gradient and, as is indicated for these transitions, a Zeeman splitting that is at least comparable with the thermal contributions to the breadths of the spectral lines. The resulting spectral features are similar in appearance, including the presence of large net circular polarization and narrow line breadths, to the commonly observed spectra of OH masers in molecular clouds. The calculations presented here are performed as a function of frequency without making the approximations of a large velocity gradient. Rapid cross relaxation, which has been advocated by others for the OH masers, is assumed.

Nedoluha, Gerald E.↗
NASA NTRS1991

An experimental comparison of nonswirling and swirling flow in a circular-to-rectangular transition duct

Circular-to-rectangular transition ducts are used as exhaust system components of aircraft with rectangular exhaust nozzles. Often, the incoming flow of these transition ducts includes a swirling velocity component remaining from the gas turbine engine. Previous transition duct studies have either not included inlet swirl or when inlet swirl was considered, only overall performance parameters were evaluated. Circular-to-rectangular transition duct flows with and without inlet swirl were explored in order to understand the effect of inlet swirl on the transition duct flow field and to provide detailed duct flow data for comparison with numerical code predictions. A method was devised to create a swirling, solid body rotational flow with minimal associated disturbances. Coefficients based on velocities and total and static pressures measured incross stream planes at four axial locations within the transition duct, along with surface static pressure measurements and surface oil film visualization, are presented for both nonswirling and swirling incoming flow. In both cases the inlet centerline Mach number was 0.35. The Reynolds number based on the inlet centerline velocity and duct inlet diameter was 1,547,000 for nonswirling and 1,366,000 for swirling flow. The maximum swirl angle was 15.6 deg. Two pair of counter-rotating side wall vortices appeared in the duct flow without inlet swirl. These vortices were absent in the swirling incoming flow cases.

Reichert, B. A.↗
NASA NTRS1991

Transonic shock-induced dynamics of a flexible wing with a thick circular-arc airfoil

Transonic shock boundary layer oscillations occur on rigid models over a small range of Mach numbers on thick circular-arc airfoils. Extensive tests and analyses of this phenomena have been made in the past but essentially all of them were for rigid models. A simple flexible wing model with an 18 pct. circular arc airfoil was constructed and tested in the Langley Transonic Dynamics Tunnel to study the dynamic characteristics that a wing might have under these circumstances. In the region of shock boundary layer oscillations, buffeting of the first bending mode was obtained. This mode was well separated in frequency from the shock boundary layer oscillations. A limit cycle oscillation was also measured in a third bending like mode, involving wind vertical bending and splitter plate motion, which was in the frequency range of the shock boundary layer oscillations. Several model configurations were tested, and a few potential fixes were investigated.

Bennett, Robert M.↗
NASA NTRS1991

Rotatable non-circular forebody flow controller

The invention is a rotatable, non-circular forebody flow controller. The apparatus comprises a small geometric device located at a nose of a forebody of an aircraft and a non-circular cross-sectional area that extends toward the apex of the aircraft. The device is symmetrical about a reference plane and preferably attaches to an axle which in turn attaches to a rotating motor. The motor rotates the device about an axis of rotation. Preferably, a control unit connected to an aircraft flight control computer signals to the rotating motor the proper rotational positioning of the geometric device.

Moskovitz, Cary A.↗
NASA NTRS1990

A linear shock cell model for non-circular jets using conformal mapping with a pseudo-spectral hybrid scheme

The shock structure in non-circular supersonic jets is predicted using a linear model. This model includes the effects of the finite thickness of the mixing layer and the turbulence in the jet shear layer. A numerical solution is obtained using a conformal mapping grid generation scheme with a hybrid pseudo-spectral discretization method. The uniform pressure perturbation at the jet exit is approximated by a Fourier-Mathieu series. The pressure at downstream locations is obtained from an eigenfunction expansion that is matched to the pressure perturbation at the jet exit. Results are presented for a circular jet and for an elliptic jet of aspect ratio 2.0. Comparisons are made with experimental data.

Bhat, Thonse R. S.↗
NASA NTRS1991

Effect of convective Mach number on mixing of coaxial circular and rectangular jets

Rectangular supersonic free and coaxial jets were used to enhance mixing relative to a circular jet in a convective Mach number range of 0.5 to 2.2. The different convective Mach numbers were obtained by changing the central jet gas composition, the temperatures of the inner and outer flows, and the velocity of the coaxial flow. The experimental techniques used were schlieren photography, total pressure, and gas-sampling measurements. For all test conditions the rectangular jets showed substantial improved mixing relative to a circular jet. The free jets showed high mixing in the circumferential region of the jet while the coaxial jet had a high mixing rate inside the central jet.

Gutmark, E.↗
NASA NTRS1991

An experimental comparison of nonswirling and swirling flow in a circular-to-rectangular transition duct

Circular-to-rectangular transition duct flows with and without inlet swirl were investigated experimentally in order to determine the effect of inlet swirl on the transition duct flow field and to provide detailed duct flow data for comparison with numerical code predictions. Coefficients based on detailed measurements of velocity, total pressure and static pressure, acquired in four cross stream planes within a circular-to-rectangular transition duct, with and without inlet swirl, are presented, as are surface static pressure and surface oil film visualization results.

Reichert, B. A.↗
NASA NTRS1991

Circular polarization in 1612 MHz OH maser emission from OH/IR stars

In order to study the incidence of circular polarization in the 1612 MHz OH emission from envelopes of OH/IR stars, the Arecibo radio telescope was used to obtain high-resolution, high signal-to-noise 1612 MHz spectra in both circular polarizations for several OH/IR stars. The method of Troland and Heiles (1982) was used to estimate the intensities of magnetic fields in the envelopes. The fields appear to have complex structures, and intensities on the order of 1-100 microG. The spectra are weakly polarized, probably as a result of weakly polarized emission from individual emitting elements. Integration over the Stokes parameter V yields nonzero results for three of the stars, suggesting that another polarizing mechanism (in addition to simple Zeeman splittings) is at work.

Zell, Philip↗
NASA NTRS1991

Linear and circular digital spectral analysis of serial data

Two methods of digital spectral analysis of unevenly sampled data are developed and illustrated here. One method uses a linear function of time (or space), the other uses circular functions. The circular method turns out to be essentially equivalent to a least-squares sine-wave analysis. The linear, anharmonic method uses only the field of real numbers and elementary algebraic operations, and hence it can be made computationally very fast and accurate. Both methods are very general, properly handling all kinds of time series ranging from simple series consisting only of the times of events to complicated series consisting of pulses with long duty cycles. The two methods are here applied to the analysis of annual mean relative sunspot numbers.

Stothers, Richard B.↗
NASA NTRS1991

Flow visualization and computational studies of a reverse flow circular combustor

The development of a reverse-flow circular combustor is described in terms of the visualization techniques that are employed to optimize the flow and mixing of fuel and air. The Fluent computer program is used in conjunction with flow visualization studies to investigate the performance of the combustor. The combustor has multiple mini-combustion zones (MCZs) and fuel injection nozzles along its circular geometry, and the fuel injection design is modeled to determine temperature distribution, including the performance at the combustor exit. The yellow dye smoke used in the visualization demonstrated desirable flow patterns with rapid circulation patterns within the MCZs and extensive interaction between the channel flow and the MCZs. The results are similar to those of CFD models, and the main conclusions are that fuel atomization can be further optimized and that the fuel injector nozzle can control swirls, fuel spray angle, and jet penetration.

Ghorashi, B.↗
NASA NTRS1991

Supersonic combustion with transverse, circular wall jets

A series of experiments are reported using inclined circular wall jets in rectangular scramjet combustion chambers configured as constant area ducts. A free piston shock tunnel was used to create inlet flow Mach numbers between 4.2 and 5.5, at enthalpies corresponding to flight speeds of between 3.5 and 5.3 km/s. Measurements of duct static pressure levels indicated that significant mixing and combustion was achieved. Using hydrogen as a fuel, it was found that multipoint injection from two sides of the duct gave a larger, and more rapid, pressure rise. In hypersonic flow, it was found that combustion only occurred for the same duct dimensions and operating conditions that supported combustion with central injection from a two-dimensional strut. Wall injection from discrete orifice circular jets was found to be more effective than two-dimensional wall injection across the whole width of the duct.

Morgan, R. G.↗
NASA NTRS1992

Reconfiguring the RUM experiment to test circular scanning with rotating unbalanced-mass devices on gimbaled payloads

This paper describes a ground-based experiment designed to prove the concept of circular scanning a gimbaled payload with rotating unbalanced-mass (RUM) devices. The experiment is a modified version of a similar experiment which demonstrates line and raster scanning with RUM's. In this paper, a description of the experiment hardware is presented and a detailed design of the servos used in scanning is given. A computer simulation model of the entire system is discussed and simulation results are included. These verify the servo designs and show the RUM's to be an extremely power efficient method for circular scanning.

Polites, M. E.↗
NASA NTRS1991

Nonlinear response of infinitely long circular cylindrical shells to subharmonic radial loads

The nonlinear response of infinitely long circular cylindrical shells (thin circular rings) in the presence of a two-to-one internal (autoparametric) resonance to a subharmonic excitation of order one-half of the higher mode is analyzed with the multiple-scale method. Four autonomous first-order ordinary differential equations are derived for the modulation of the amplitudes and phases of the interacting models. These modulation equations are used to determine the fixed points and their stability. The fixed points correspond to periodic oscillations of the shell, whereas the limit-cycle solutions of the modulation equations correspond to amplitude and phase-modulated oscillations of the shell. The force response curves exhibit saturation, jumps, and Hopf bifurcation. As excitation frequency changes, all limit cycles deform and lose stability through either pitchfork or cyclic-fold (saddle-node) bifurcations. Some of these saddle-node bifurcations cause a transition to chaos. The pitchfork bifurcations break the symmetry of the limit cycles.

Nayfeh, Ali H.↗
NASA NTRS1993

Scattering-Matrix Program For Circular Waveguide Components

Scattering Matrix Program for Circular Waveguide Junctions, CWGSCAT, computes scattering matrices of series of circular waveguide sections. Sections must have same axis, but radius and length of each section completely arbitrary. Step discontinuities, corrugated sections, and horns analyzed. Written in FORTRAN.

Hoppe, Daniel J.↗